1. Field of the Invention
The present invention relates generally to the field of diagnostic radiography and, more particularly, to an anti-scatter grid capable of yielding high resolution, high contrast radiographic images.
2. Description of the Related Art
During medical diagnostic radiography processes, x-radiation impinges upon a patient. Some of the x-radiation becomes absorbed by the patient's body, and the remainder of the x-radiation penetrates through the body. The o differential absorption of the x-radiation permits the formation of a radiographic image on a photosensitive film.
Of the x-rays that pass through the body, primary radiation travels unimpeded and directly along the path from which the x-rays were originally emitted from the source. Scattered radiation is that which passes through the body, is scattered by the body elements, and thus travels at an angle from the original path. Both primary and scattered radiation will expose a photosensitive film, but scattered radiation, by nature of its trajectory, reduces the contrast (sharpness) of the projected image. In conventional posterior/anterior chest x-ray examinations, for example, about sixty percent of the radiation that penetrates through the body can be in the form of scattered radiation and thus impart a significant loss of image contrast. Therefore, it is desirable to filter out as much of the scattered radiation as possible.
One embodiment for filtering scattered radiation includes an anti-scatter grid which is interposed between the body and the photosensitive film. Scattered radiation impinges upon absorbent (opaque) material in the grid and becomes absorbed. Also absorbed by the absorbing material, however, is a portion of the primary radiation. The radiographic imaging arrangement of this embodiment provides higher contrast radiographs by virtue of the elimination of the scattered radiation, but necessitates an increase in radiation dosage to the patient in order to properly expose the photographic element. The increased radiation requirement results in part because the scattered radiation no longer constitutes part of the imaging x-ray beam, and in part because as much as 30% or more of the primary beam impinges upon the absorbing material in the grid and itself becomes filtered out (i.e. absorbed).
The increased radiation required for the exposure can be a factor of seven (7) or more, i.e., the patient can receive seven times the x-radiation dose when the grid is used as a part of the radiographic system. Because high doses of x-radiation pose a health hazard to the exposed individual, there has been a continual need to reduce the amount of x-radiation a patient receives during the course of a radiographic examination.
Many conventional grids use thin lead strips as the x-ray absorber and either aluminum strips or fiber composite strips as transparent interspace material. Conventional manufacturing processes consist of tediously laminating individual strips of the absorber material and non-absorber interspace material by laboriously gluing together alternate layers of the strips until thousands of such alternating layers comprise a stack. Furthermore, to fabricate a focused grid, the individual layers must be placed in a precise manner so as to position them at a slight angle to each other such that each layer is fixedly focused to a convergent line: the x-ray source. After the composite of strips is assembled into a stack, it must then be cut and carefully machined along its major faces to the required grid thickness that may be as thin as only 0.5 millimeters, the fragile composite then being, for example, 40 cm by 40 cm by 0.5 mm in dimension and very difficult to handle. If the stack has survived the machining and handling processes, the stack must further be laminated with sufficiently strong materials so as to reinforce the fragile grid assembly and provide enough mechanical strength for use in the field. Accidental banging, bending, or dropping of such grids can cause internal damage, i.e., delamination of the layers which cannot be repaired, rendering the grid completely useless.
A significant parameter in the grid design is the grid ratio, which is defined as the ratio between the height of the x-ray absorbing strips and the distance between them. The ratios typically range from 4:1 to 16:1. Because a value of about 0.050 mm lead thickness is a practical lower limit imposed by current manufacturing limitations, i.e., it being extremely difficult to handle strips at this thickness or thinner, a grid with a ratio of 4:1 with a line rate of 60 lines per centimeter demands that the interspace material be 0.12 mm in thickness and results in a grid that is only 0.5 mm thick. Because of the manufacturing limitations, the lead strips in these grids are generally too wide and, consequently, yield a large cross-sectional area that undesirably absorbs as much as 30% or more of the primary radiation. Furthermore, the thick strips result in an undesirable shadow-image cast onto the film. To obliterate the shadows, it becomes necessary to provide a mechanical means for moving the grid during the exposure period. This motion of the grid causes lateral decentering and can consequently result in absorption of an additional 20% of the primary radiation. Thus the use of wide absorber strips requires a significant increase in patient dosage to compensate this drawback.